[Shotwell] Basic face recognition working!

Hi,

I managed to get a basic version of face recognition working in my branch.

It uses the OpenFace DNN model I had described earlier.

There are some optimizations to be done for storing the intermediate face representation vector.

The UI may also need improvement. I added a 'Train Face from Photo' menu item in the FacePage menu for now.

A face from a single photo is used as reference to compare with unknown faces in other photos when running detection.

I also modified the DBus interface to do both face boundary detection and vector generation in the same call.

More feedback is welcome.

Thanks,

Narendra